Tone frequency track circuits – RCs with signal current in the tonal frequency range, operating on a frequency amplitude-modulated signal.

The concept of “tonal track circuits” appeared in 1990, although auto-locks (AB) on the basis of shopping and entertainment centers began to be used in Russia and the CIS in 1985. However, it was the creation of various ABs, both with the BRC (continuous rail circuit) and with insulating joints, which made it necessary to combine these RCs into one class.

The SEC scheme is based on the classical scheme of a continuous rail circuit, the feature of which is the power supply of one source – the generator – 2 adjacent RCs.

The tonal frequency track circuits of the first 2 generations had a range of carrier frequencies:

I generation: 425 Hz and 475 Hz;
II generation: 575 Hz, 725 Hz, 775 Hz.
These types of RCs were used in electrified areas with a direct current and in areas with an autonomous type.

However, most railway contact networks operate on alternating current. Therefore, the first malls quickly became morally obsolete. This also contributed to the presence of areas with low ballast resistance. From 1993 to the present, shopping malls of the third and fourth generations are used – shopping and entertainment centers-3 and shopping and recreation centers-4.

Tonal track circuits of the III and IV generations are suitable for any type of thrust and can be used in areas with both normal and reduced ballast resistance. These SECs have the following range of carrier frequencies:

SEC-3: 420 Hz, 480 Hz, 580 Hz, 720 Hz, 780 Hz;
SEC-4 (high frequency SEC): 4500 Hz, 5000 Hz, 5500 Hz.
Modulation frequencies: 8 Hz and 12 Hz.

The maximum length of the SEC-3 is 1000 m. With the use of insulating joints, the length increases to 1300 m.
The maximum length of the SEC-4 is 300 m.

In addition to the above listed, the tonal rail circuits used in the microprocessor-based auto-blocking system, including the frequency range 1900 ÷ 2800 Hz, are also referred to as tonal track circuits.

Advantages of the mall
it is possible to lay an all-welded railway track between stations without using insulating joints on tracks, which account for 27% of equipment failures of ZhAT systems;
a significant reduction in the use of electrified sections of choke transformers, which contributes to reducing
• failures due to breakage and theft of bridges,
• maintenance costs;
mall equipment can be placed at the stations;
used in all types of traction;
reduction of electricity consumption;
lack of contact relays, which significantly increases the reliability and durability of the equipment;
high protection against interference from traction current.
Disadvantages SEC
low limiting length;
additional bypass zone;
in comparison with code RCs, a relatively large amount of equipment and cable consumption.
The economic viability of the SEC
It is advisable to use tone frequency track circuits:

To control the free span and the health of the rails in the semi-automatic blocking system (PAB), in addition to improving the safety of train traffic, this allows the implementation of centralized control room systems.
For equipment in code and pulse-wire AB protective sections of the required length without additional installation of relay cabinets and high-voltage linear transformers.
To obtain the required length of sections of approach to the railway crossing, which allows minimizing the prematurity of closing the crossing.
On railway sites with reduced ballast resistance.